In many situations, it can be desirable to know a location of a person. For instance, in an emergency situation, the knowledge of the location of emergency responders (e.g., firefighter, police, and/or armed forced personnel) can be useful to provide further assistance to others and/or to locate a missing and/or injured emergency responder.
Equipping emergency responders with a navigation and/or locating device can allow other responders to rapidly locate team members. Such a device, typically known as a personal navigation device, can include an inertial measurement unit (IMU) along with a Global Positioning Signal (GPS) receiver to establish an initial position of a person prior to entering a location where GPS may not be available and/or may be available intermittently, and a transmitter for relaying the position of the responder to others. A personal navigation device can be GPS aided, as the GPS signal can be used to update and correct the personal navigation device's location information whenever a GPS signal is available, thereby aiding the personal navigation device in maintaining the correct location information.
As an example, a personal navigation device can assist emergency responders, such as a firefighter, that are required to enter a building to perform rescue and/or fight a fire. The GPS signal may not generally be available in the building. Prior to entering the building, a firefighter equipped with a GPS aided personal navigation device may establish the firefighter's initial position. As the firefighter traverses the building, the firefighter's location may be accurately known at all times because in the absence of a GPS signal, the IMU can track the firefighter's motion. In the course of the firefighter exploring the building, if the firefighter discovers people in need of assistance, the firefighter can call additional responders to the firefighter's location and the responders will know immediately where the firefighter is and/or how to get to the firefighter.
Without such a device, the firefighter may be required to provide a detailed description of where the firefighter is and/or how to get to the firefighter. If the firefighter is hurt, the firefighter may be unable to communicate the firefighter's location and the remainder of the firefighter's team may have to search the building to locate the firefighter, as well as any other victim.
A severe problem for devices of this type is that acquiring a GPS position fix does not take place immediately upon the GPS receiver being exposed to GPS signals. The lag between turn on and acquisition of the current position is known as “time to first fix”. Depending on conditions, this delay could be as small as 10-15 seconds or as large as several minutes. When a GPS receiver is turned on, the farther the GPS receiver is from the last position, the longer it takes to acquire the current position. Because some vehicle bodies interfere with access to GPS signals for their occupants, a first responder traveling to a scene may not be able to establish his position until the first responder exits the vehicle. Without some sort of assistance, a first responder may find that it takes longer for the GPS to establish a position after exiting the vehicle than the time for the first responder to enter a building with minimal or no GPS signal availability.